Hitherto, in a vehicle failure diagnostic device, there is a device for storing, when a failure occurs, various types of data in a predetermined period from a time point of the failure and subsequently reading and analyzing the data, thereby recognizing a state of the vehicle at the time of the failure occurrence, a cause of the failure, and the like. Moreover, the amount of the stored data may become excessively large, and hence a storage device is configured to store the data only for the predetermined period without storing the other data, thereby restricting the data amount (refer to Patent Literature 1).